diff --git a/Lib/fontTools/merge.py b/Lib/fontTools/merge.py
index d7d5fa3..38db704 100644
--- a/Lib/fontTools/merge.py
+++ b/Lib/fontTools/merge.py
@@ -418,7 +418,7 @@
 ttLib.getTableClass('JSTF').mergeMap = \
 ttLib.getTableClass('MATH').mergeMap = \
 {
-	'tableTag': equal,
+	'tableTag': implementedFilter(equal), # XXX clean me up
 	'table': mergeObjects,
 }